The explosive growth of mobile data demands tremendous growth of mobile service coverage and network capacity. A promising solution is to offload data traffic to small cells, such as micro cells, pico cells and femto cells. The small cells can be deployed at hotspot areas within a cellular macro cell. Small cells provide low power, low cost and efficient connectivity and coverage for all users. The increasing popularity of smart phones and tablet also drives the exponential growth in small cell deployment.
One of the main issue with the exponential growth of small cells deployment is energy saving. The traffic pattern of a small cell may fluctuate sharply. A small cell designed to support a peak traffic is inevitably under used when the traffic dramatically reduced or even disappeared. For example, the traffic of a small cell serving a conference room is designed to support a large number of users during a meeting. The traffic needs of this small cell may be dramatically reduced during other time. With the exponential growth of the small cell deployment, it is important to address the energy saving issue for the small cells when underutilized.
One solution is network-based energy saving for small cells. This approach has low impact on base stations. However, it has high network impact because it involves high complexity OAM implementation and increases costly wireless core network overhead. Further, costly network upgrade makes such centralized solution hard to make dynamic changes. As the technology for small cell and other wireless is evolving rapidly, these shortcomings make such solution less appealing.
The other solution to the energy saving for small cells is signal-based solution. Its major impact is on base stations. However, it offers very low core network impact and requires few core network overhead. Further, signal-based solution offers more flexibility for dynamic changes. Therefore, signal-based energy saving for small cells is a preferred solution.
Though signal-based solution has advantages over the network-based solutions, several problems exist. Small cell energy saving requires switching off underutilized small cells and switching on neighboring small cells to offload traffic when needed. The problem with the existing switching off scheme occurs when two or more neighboring small cells switching off independently upon detecting the low traffic load of their own. Switching-off small cells hand off their traffic to the neighboring small cells. When multiple small cells switching off at the about the same time, the traffic load to the neighboring small cells increases dramatically, resulting in the neighboring cell overloading. Problems exist for the current switch-on scheme as well. The first problem is activating unhelpful small cells. Upon detecting a high traffic load, the small cell broadcast the switch-on request to its neighboring cells, resulting in unnecessarily activating small cells. The second problem is the small cells switches on upon receiving switch-on request from neighbor cells. Upon switching on, the small cell determines that the traffic load is below the low traffic threshold triggering the switch off procedures. During switching off, the traffic is handed over to the previous small cell resulting in the higher than the high-threshold traffic. Such ping-pong effect makes the current switch-on scheme problematic as well.
Improvements and enhancements are required to provide more efficient and reliable energy saving method for small cells.